    Patent document 1: Publication No. 1999-103737 (Japanese Patent Application No. 1997-272978)
Characteristics required from, for example, a fishing line include light weight, high strength, and durability as these properties.
With regard to high strength, which a nylon monofilament lacks as a disadvantageous point of a conventionally used fishing line, various types of covered yarns formed by winding a sheath yarn around a core yarn have been proposed to satisfy the object of high strength.
For example, the fishing line according to patent document 1 has been proposed with an object to satisfy the high strength, without compromising production efficiency and abrasion resistance of the fishing line.
The fishing line is a covered yarn which adopts a synthetic fiber multifilament yarn as the core yarn, around which a twisted synthetic fiber multifilament yarn is wound as the sheath yarn, such that a difference between the angle defined by the core yarn and the sheath yarn and the twisting angle of the sheath yarn does not exceed 25°.
The cited invention claims, with such a structure, that it provides a fishing line having excellent mechanical characteristics such as high breaking strength, high knot strength, low breaking elongation, and also high abrasion resistance.
However, as a trade-off with the high strength attained with the multifilament (twisted yarn), the fishing line often spins and becomes tangled with the guide of the fishing rod, when fishing using the twisted yarn.
One of the present inventors (Yasuhisa Yamamoto) has created a new invention (Japanese Patent Application No. 2005-56927), with an object to solve the foregoing problem of the fishing line according to patent document 1, and further to improve the tensile strength thereof. The invention provides a fishing line including a plurality of multifilament yarns, and formed by ply-twisting the multifilament yarns such that a value obtained by dividing a value twice as large as the ratio of number of twists between a preliminary twist and a final twist, by the number of multifilament yarns to be twisted together becomes not less than 1.2 and not more than 2.5. Such an invention restricts the fishing line from spinning and thereby from becoming tangled with the guide of the fishing rod, thus significantly minimizing the trouble in use, when fishing with the fishing line made of the multifilament yarn (twisted yarn).
On the other hand, the present inventor has constantly studied, even after submitting the foregoing invention, on the possibility of further improvement in knot strength of the yarn.
The intended improvement in knot strength would provide enormous benefit not only to the fishing line but also to yarn products in general, such as the gut strings of a tennis racket, embroidery threads, sewing threads, ropes and yarns used as the core of a curl-cord strap for a mobile phone, as well as yarn used for manufacturing fishing nets and safety nets.
Regarding the fishing line in particular, although the knot strength can be improved, if the circularity is degraded (flattening ratio is increased), in other words if the cross-section of the fishing line becomes flattened, the fishing line unnaturally reflects light, thereby causing the fish to have a sense of caution. Also, the degradation in circularity may lead to degradation in tensile strength (gross knot strength), although the knot strength is improved. Further the degradation in circularity often provokes such line trouble as becoming tangled with the guide of the fishing rod, as described above.